Anastasios PAPAZAFEIROPOULOS | Imperial College London (original) (raw)

Papers by Anastasios PAPAZAFEIROPOULOS

IEEE Access

Terahertz (THz) band is expected to be one of the key enabling technologies of the sixth generati... more Terahertz (THz) band is expected to be one of the key enabling technologies of the sixth generation (6G) wireless networks because of its abundant available bandwidth and very narrow beamwidth. Due to high frequency operations, electrically small array apertures are employed, and the signal wavefront becomes spherical in the near-field. Therefore, near-field signal model should be considered for channel acquisition in THz systems. Unlike prior works which mostly ignore the impact of near-field beam-squint (NB) and consider either narrowband scenario or far-field models, this paper introduces both a model-based and a model-free techniques for wideband THz channel estimation in the presence of NB. The model-based approach is based on orthogonal matching pursuit (OMP) algorithm, for which we design an NB-aware dictionary. The key idea is to exploit the angular and range deviations due to the NB. We then employ the OMP algorithm, which accounts for the deviations thereby ipso facto mitigating the effect of NB. We further introduce a federated learning (FL)-based approach as a model-free solution for channel estimation in a multiuser scenario to achieve reduced complexity and training overhead. Through numerical simulations, we demonstrate the effectiveness of the proposed channel estimation techniques for wideband THz systems in comparison with the existing state-of-the-art techniques. INDEX TERMS Beamsquint, channel estimation, federated learning, machine learning, near-field, orthogonal matching pursuit, sparse recovery, terahertz.

IEEE Open Journal of the Communications Society

For the demonstration of ultra-wideband bandwidth and pencil-beamforming, the terahertz (THz)-ban... more For the demonstration of ultra-wideband bandwidth and pencil-beamforming, the terahertz (THz)-band has been envisioned as one of the key enabling technologies for the sixth generation networks. However, the acquisition of the THz channel entails several unique challenges such as severe path loss and beam-split. Prior works usually employ ultra-massive arrays and additional hardware components comprised of time-delayers to compensate for these loses. In order to provide a cost-effective solution, this paper introduces a sparse-Bayesian-learning (SBL) technique for joint channel and beam-split estimation. Specifically, we first model the beam-split as an array perturbation inspired from array signal processing. Next, a low-complexity approach is developed by exploiting the line-of-sight-dominant feature of THz channel to reduce the computational complexity involved in the proposed SBL technique for channel estimation (SBCE). Additionally, based on federated-learning, we implement a model-free technique to the proposed model-based SBCE solution. Further to that, we examine the near-field considerations of THz channel, and introduce the range-dependent near-field beam-split. The theoretical performance bounds, i.e., Cramér-Rao lower bounds, are derived both for near-and far-field parameters, e.g., user directions, beam-split and ranges. Numerical simulations demonstrate that SBCE outperforms the existing approaches and exhibits lower hardware cost. INDEX TERMS Terahertz, channel estimation, beam split, sparse Bayesian learning, near-field, federated learning.

IEEE Transactions on Green Communications and Networking

In the current contribution, we examine the feasibility of fully-energy-autonomous operation of r... more In the current contribution, we examine the feasibility of fully-energy-autonomous operation of reconfigurable intelligent surfaces (RIS) through wireless energy harvesting (EH) from incident information signals. Towards this, we first identify the main RIS energy-consuming components and present a suitable and accurate energy-consumption model that is based on the recently proposed integrated controller architecture and includes the energy consumption needed for channel estimation. Building on this model, we introduce a novel RIS architecture that enables EH through RIS unit-cell (UC) splitting. Subsequently, we introduce an EH policy, where a subset of the UCs is used for beamsteering, while the remaining UCs absorb energy. In particular, we formulate a subset al.ocation optimization problem that aims at maximizing the signal-to-noise ratio (SNR) at the receiver without violating the RIS's energy consumption demands. As a problem solution, we present low-complexity heuristic algorithms. The presented numerical results reveal the feasibility of the proposed architecture and the efficiency of the presented algorithms with respect to both the optimal and very high-complexity

2017 IEEE International Conference on Communications (ICC), 2017

This work encompasses Rate-Splitting (RS), providing significant benefits in multiuser settings i... more This work encompasses Rate-Splitting (RS), providing significant benefits in multiuser settings in the context of huge degrees of freedom promised by massive Multiple-Input Multiple-Output (MIMO). However, the requirement of massive MIMO for cost-efficient implementation makes them more prone to hardware imperfections such as phase noise (PN). As a result, we focus on a realistic broadcast channel with a large number of antennas and hampered by the unavoidable PN. Moreover, we employ the RS transmission strategy, and we show its robustness against PN, since the sum-rate does not saturate at high signal-tonoise ratio (SNR). Although, the analytical results are obtained by means of the deterministic equivalent analysis, they coincide with simulation results even for finite system dimensions.

IEEE Wireless Communications Letters, 2021

In this paper, we focus on the coverage probability of a double-intelligent reflecting surface (I... more In this paper, we focus on the coverage probability of a double-intelligent reflecting surface (IRS) assisted wireless network and study the impact of multiplicative beamforming gain and correlated Rayleigh fading. In particular, we obtain a novel closed-form expression of the coverage probability of a single-input single-output (SISO) system assisted by two large IRSs while being dependent on the corresponding arbitrary reflecting beamforming matrices (RBMs) and large-scale statistics in terms of correlation matrices. Taking advantage of the largescale statistics, i.e., statistical channel state information (CSI), we perform optimization of the RBMs of both IRSs once per several coherence intervals rather than at each interval. Hence, we achieve a reduction of the computational complexity, otherwise increased in multi-IRS-assisted networks during their RBM optimization. Numerical results validate the analytical expressions even for small IRSs, confirm enhanced performance over the conventional single-IRS counterpart, and reveal insightful properties.

This paper investigates the achievable sum-rate of massive multiple-input multiple-output (MIMO) ... more This paper investigates the achievable sum-rate of massive multiple-input multiple-output (MIMO) systems in the presence of channel aging. For the uplink, by assuming that the base station (BS) deploys maximum ratio combining (MRC) or zero-forcing (ZF) receivers, we present tight closed-form lower bounds on the achievable sum-rate for both receivers with aged channel state information (CSI). In addition, the benefit of implementing channel prediction methods on the sum-rate is examined, and closed-form sum rate lower bounds are derived. Moreover, the impact of channel aging and channel prediction on the power scaling law is characterized. Extension to the downlink scenario and multi-cell scenario are also considered. It is found that, for a system with/without channel prediction, the transmit power of each user can be scaled down at most by 1/ √ M (where M is the number of BS antennas), which indicates that aged CSI does not degrade the power scaling law, and channel prediction does...

IEEE Wireless Communications Letters, 2021

This paper investigates the impact of spatial channel correlation on the outage probability of in... more This paper investigates the impact of spatial channel correlation on the outage probability of intelligent reflecting surface (IRS)-assisted single-input single-output (SISO) communication systems. In particular, we derive a novel closed-form expression of the outage probability for arbitrary phase shifts and correlation matrices of the indirect channels. To shed light on the impact of the spatial correlation, we further attain the closedform expressions for two common scenarios met in the literature when the large-scale fading coefficients are expressed by the loss over a propagation distance. Numerical results validate the tightness and effectiveness of the closed-form expressions. Furthermore, the spatial correlation offers significant decreases in the outage probability as the direct channel is blocked.

2020 IEEE 31st Annual International Symposium on Personal, Indoor and Mobile Radio Communications, 2020

Despite the deleterious effect of hardware impairments (HWIs) on wireless systems, most prior wor... more Despite the deleterious effect of hardware impairments (HWIs) on wireless systems, most prior works in cell-free (CF) massive multiple-input-multiple-output (mMIMO) systems have not accounted for their impact. In particular, the effect of phase noise (PN) has not been investigated at all in CF systems. Moreover, there is no work investigating HWIs in scalable CF (SCF) mMIMO systems, encountering the prohibitively demanding fronthaul requirements of large networks with many users. Hence, we derive the uplink spectral efficiency (SE) under HWIs with minimum mean-squared error (MMSE) combining in closedform by means of the deterministic equivalent (DE) analysis. Notably, previous works, accounted for MMSE decoding, studied the corresponding SE only by means of simulations. Numerical results illustrate the performance loss due to HWIs and result in insightful conclusions. Index Terms-Cell-free massive MIMO systems, user-centric 5G networks, transceiver hardware impairments, MMSE processing, achievable spectral efficiency.

IEEE Transactions on Green Communications and Networking, 2021

Motivated by the ever-growing demand for green wireless communications and the advantages of cell... more Motivated by the ever-growing demand for green wireless communications and the advantages of cell-free (CF) massive multiple-input multiple-output (MIMO) systems, we focus on the design of their downlink for optimal energy efficiency (EE). To address this fundamental topic, we assume that each access point (AP) is deployed with multiple antennas and serves multiple users on the same time-frequency resource while the APs are Poisson point process (PPP) distributed, which approaches realistically their opportunistic spatial randomness. Relied on tools from stochastic geometry, we derive a lower bound on the downlink average achievable spectral efficiency (SE). Next, we consider a realistic power consumption model for CF massive MIMO systems. These steps enable the formulation of a tractable optimization problem concerning the downlink EE per unit area, which results in the analytical determination of the optimal pilot reuse factor, the AP density, and the number of AP antennas and users that maximize the EE. Notably, the EE per unit area and not just the EE is the necessary metric to describe CF systems, where we meet multi-point transmission. Hence, we provide useful design guidelines for CF massive MIMO systems relating to fundamental system variables towards optimal EE. Among the results, we observe that a lower pilot reuse factor enables a decrease of the interference, and subsequently, higher EE up to a specific value. Overall, it is shown that the CF massive MIMO technology is a promising candidate for next-generation networks achieving simultaneously high SE and EE per unit area.

IEEE Transactions on Vehicular Technology, 2017

Rate-Splitting (RS) has recently been shown to provide significant performance benefits in variou... more Rate-Splitting (RS) has recently been shown to provide significant performance benefits in various multiuser transmission scenarios. In parallel, the huge degrees-of-freedom provided by the appealing massive Multiple-Input Multiple-Output (MIMO) necessitate the employment of inexpensive hardware, being more prone to hardware imperfections, in order to be a cost-efficient technology. Hence, in this work, we focus on a realistic massive Multiple-Input Single-Output (MISO) Broadcast Channel (BC) hampered by the inevitable hardware impairments. We consider a general experimentally validated model of hardware impairments, accounting for the presence of multiplicative distortion due to phase noise, additive distortion noise and thermal noise amplification. Under both scenarios with perfect and imperfect channel state information at the transmitter (CSIT), we analyze the potential robustness of RS to each separate hardware imperfection. We analytically assess the sum-rate degradation due to hardware imperfections. Interestingly, in the case of imperfect CSIT, we demonstrate that RS is a robust strategy for multiuser MIMO in the presence of phase and amplified thermal noise, since its sum-rate does not saturate at high signal-to-noise ratio (SNR), contrary to conventional techniques. On the other hand, the additive impairments always lead to a sum-rate saturation at high SNR, even after the application of RS. However, RS still enhances the performance. Furthermore, as the number of users increases, the gains provided by RS decrease not only in ideal conditions, but in practical conditions with RTHIs as well. Notably, although a deterministic equivalent analysis is employed, the analytical and simulation results coincide even for finite system dimensions. As a consequence, the applicability of these results also holds for current "small-scale" multi-antenna systems. Index Terms-Rate-splitting, massive MIMO, residual hardware impairments, regularized zero-forcing precoding, deterministic equivalent analysis.

IEEE Transactions on Vehicular Technology, 2017

Despite the inevitable presence of transceiver impairments, most prior work on multiple-input mul... more Despite the inevitable presence of transceiver impairments, most prior work on multiple-input multiple-output (MIMO) wireless systems assumes perfect transceiver hardware which is unrealistic in practice. In this direction, motivated by the increasing interest in MIMO relay systems due to their improved spectral efficiency and coverage, this paper investigates the impact of residual hardware impairments on the ergodic capacity of dual-hop (DH) amplify-and-forward (AF) MIMO relay systems. Specifically, a thorough characterization of the ergodic channel capacity of DH AF relay systems in the presence of hardware impairments is presented herein for both the finite and large antenna regimes by employing results from finite-dimensional and large random matrix theory, respectively. Regarding the former setting, we derive the exact ergodic capacity as well as closed-form expressions for tight upper and lower bounds. Furthermore, we provide an insightful study for the low signalto-noise ratio (SNR) regimes. Next, the application of the free probability (FP) theory allows us to study the effects of the hardware impairments in future 5G deployments including a large number of antennas. While these results are obtained for the large system limit, simulations show that the asymptotic results are quite precise even for conventional system dimensions.

2016 IEEE 17th International Workshop on Signal Processing Advances in Wireless Communications (SPAWC), 2016

This paper aims at a realistic evaluation of Rayleighproduct multiple-input multiple-output (MIMO... more This paper aims at a realistic evaluation of Rayleighproduct multiple-input multiple-output (MIMO) systems. Specifically, by considering the residual transceiver hardware impairments into account, we derive the ergodic channel capacity of a MIMO system with optimal receivers in the case of insufficient scattering. Actually, motivated by the increasing interest for massive MIMO systems, we investigate the impact of transceiver hardware imperfections in systems with both finite (conventional) and large number of antennas under rank deficient channel matrix conditions by varying the severity of hardware quality. Among the interesting outcomes, we emphasize that the residual hardware transceiver impairments result to a saturation of the ergodic channel capacity within the high signal-to-noise ratio (SNR) regime. Furthermore, we observe that the higher the "richness" of the scattering environment, the higher the ergodic channel capacity till it gets saturated.

2016 IEEE International Conference on Communications Workshops (ICC), 2016

The emerging requirements of the fifth generation (5G) wireless communications are high spectral ... more The emerging requirements of the fifth generation (5G) wireless communications are high spectral efficiency, low latency, and ubiquitous coverage. In this direction, Dual-Hop (DH) Amplify-and-Forward (AF) relaying has been widely investigated due to its simplicity, low implementation complexity and low transmission delay. However, most existing works assume ideal transceiver hardware which is impractical. In practice, a cost-efficient wireless transceiver has to combat the effects of several inevitable impairments such as high power amplifier nonlinearities, In-phase/Quadrature-phase (I/Q)-imbalance, and oscillator phase noise, which can be only partially compensated using calibration algorithms. In this direction, this paper analyzes the Minimum Mean Square Error (MMSE) filtering performance of a DH-AF Multiple-Input-Multiple-Output (MIMO) wireless system considering the effects of the residual additive impairments at the transmitter and receiver of both hops. Using free probability principles, the MMSE filtering performance of the considered system is studied and a tight lower bound is proposed by taking the effects of residual additive transceiver impairments into account. Our numerical results show that the MMSE filtering performance of the DH-AF massive MIMO relay system significantly degrades and results to saturation in the presence of residual additive transceiver impairments.

IEEE Transactions on Vehicular Technology, 2017

In this paper, we analyze the performance of the uplink communication of massive multi-cell multi... more In this paper, we analyze the performance of the uplink communication of massive multi-cell multiple-input multipleoutput (MIMO) systems under the effects of pilot contamination and delayed channels because of terminal mobility. The base stations (BSs) estimate the channels through the uplink training, and then use zero-forcing processing to decode the transmit signals from the users. The probability density function (PDF) of the signal-to-interference-plus-noise ratio is derived for any finite number of antennas. From this PDF, we derive an achievable ergodic rate with a finite number of BS antennas in closed form. Insights of the impact of the Doppler shift (due to terminal mobility) at the low signal-to-noise ratio regimes are exposed. In addition, the effects on the outage probability are investigated. Furthermore, the power scaling law and the asymptotic performance result by infinitely increasing the numbers of antennas and terminals (while their ratio is fixed) are provided. The numerical results demonstrate the performance loss for various Doppler shifts. Among the interesting observations revealed is that massive MIMO is favorable even in channel aging conditions. Index Terms-Delayed channels, massive MIMO, multiuser MIMO system, zero-forcing processing.

2015 IEEE Globecom Workshops (GC Wkshps), 2015

Application of multiple-input and multiple-output (MIMO) technology in wireless systems not only ... more Application of multiple-input and multiple-output (MIMO) technology in wireless systems not only provides high spectral efficiency by the deployment of many number of antennas at both sides of a communication system for a given total transmit power, but it also enhances physical-layer security. Given the need for evaluation of realistic systems, this works studies the total information flow during multicast transmission in a region R, accounting for the unavoidable statistical spatial randomness of users locations, both in absence or presence of eavesdroppers under conditions of imperfect channel state information at the transmitter (CSIT) due to users' mobility. In particular, we first introduce a lower-bound of the spacetime mutual information without any eavesdropping, and then we proceed with the derivation for both finite and infinitely large number of antennas. Moreover, in case that eavesdropping is present, we determine a metric, called space-time secrecy rate with imperfect CSIT, which characterizes the total amount of common confidential information flow per receive antenna, and we obtain it with asymptotic arguments in terms of the numbers of antennas at each side. Numerical results, validated by simulations, shed light on how the imperfect conditions affect the performance. Remarkably, imperfect CSIT may present benefits, since it engenders degradation in eavesdropper's performance.

IEEE Transactions on Wireless Communications, 2015

Delayed channel state information at the transmitter (CSIT) due to time variation of the channel,... more Delayed channel state information at the transmitter (CSIT) due to time variation of the channel, coming from the users' relative movement with regard to the BS antennas, is an inevitable degrading performance factor in practical systems. Despite its importance, little attention has been paid to the literature of multi-cellular multiple-input massive multiple-output (MIMO) system by investigating only the maximal ratio combining (MRC) receiver and the maximum ratio transmission (MRT) precoder. Hence, the contribution of this work is designated by the performance analysis/comparison of/with more sophisticated linear techniques, i.e., a minimum-mean-square-error (MMSE) detector for the uplink and a regularized zero-forcing (RZF) precoder for the downlink are assessed. In particular, we derive the deterministic equivalents of the signal-to-interference-plus-noise ratios (SINRs), which capture the effect of delayed CSIT, and make the use of lengthy Monte Carlo simulations unnecessary. Furthermore, prediction of the current CSIT after applying a Wiener filter allows to evaluate the mitigation capabilities of MMSE and RZF. Numerical results depict that the proposed achievable SINRs (MMSE/RZF) are more efficient than simpler solutions (MRC/MRT) in delayed CSIT conditions, and yield a higher prediction at no special computational cost due to their deterministic nature. Nevertheless, it is shown that massive MIMO are preferable even in time-varying channel conditions. Index Terms-Massive MIMO, delayed CSIT, channel estimation, channel prediction, linear precoding, linear detection. I. INTRODUCTION Multiuser MIMO (MU-MIMO), applied to next generation systems (e.g., 802.16m [3], LTE-Advanced [4]), is one of the core technologies promising to provide a remarkable increase in data rates. Such systems include several co-channel users communicating with a base station (BS) equipped with multiple antennas. However, the technological transition to 5G systems is expected to demand a thousand-fold higher capacity. Massive MIMO, where the BS includes a very large number of antennas, have emerged as one of the most promising technologies towards this direction because more degrees of freedom and increased power efficiency are achieved by simplifying multiuser processing, reducing transmit power, as

2013 IEEE 24th Annual International Symposium on Personal, Indoor, and Mobile Radio Communications (PIMRC), 2013

ABSTRACT We consider the two-user time correlated multiple-antenna interference channel (IC) unde... more ABSTRACT We consider the two-user time correlated multiple-antenna interference channel (IC) under realistic conditions. Specifically, the transmitters have not only perfect delayed channel state information (CSIT), but also asymmetrically imperfect current CSIT. Delayed CSIT, met to cases of practical interest (certain feedback latency and limited quality), can still be useful as shown by Maddah-Ali and Tse. In parallel, the assumption of unequal prediction of current CSIT based on past samples across different links, conduces to the consideration of a more realistic scenario, where different links face different Doppler spread. The degrees of freedom (DoF) region of the IC under these general conditions is obtained and characterized. In particular, we investigate the effects of the asymmetry appearing to different links, such as how the feedback capability of one user affects the other. In addition, the achievability schemes are provided. Finally, Monte Carlo simulation in terms of the sum rate validates the theoretical results and provides deeper insight.

2014 IEEE 25th Annual International Symposium on Personal, Indoor, and Mobile Radio Communication (PIMRC), 2014

Massive multiple-input multiple-output (MIMO) networks, where the base stations (BSs) are equippe... more Massive multiple-input multiple-output (MIMO) networks, where the base stations (BSs) are equipped with large number of antennas and serve a number of users simultaneously, are very promising, but suffer from pilot contamination. Despite its importance, delayed channel state information (CSI) due to user mobility, being another degrading factor, lacks investigation in the literature. Hence, we consider an uplink model, where each BS applies zero-forcing decoder, accounting for both effects, but with the focal point on the relative users' movement with regard to the BS antennas. In this setting, analytical closed-form expressions for the sum-rate with finite number of BS antennas, and the asymptotic limits with infinite number of BS antennas epitomize the main contributions. In particular, the probability density function of the signal-to-interference-plus-noise ratio and the ergodic sum-rate are derived for any finite number of antennas. Insights of the impact of the arising Doppler shift due to user mobility into the low signal-to-noise ratio regime as well as the outage probability are obtained. Moreover, asymptotic analysis performance results in terms of infinitely increasing number of antennas, power,

2014 IEEE 25th Annual International Symposium on Personal, Indoor, and Mobile Radio Communication (PIMRC), 2014

In this work we investigate the degrees of freedom (DoF) performance of the multiple-input multip... more In this work we investigate the degrees of freedom (DoF) performance of the multiple-input multiple-output (MIMO) interference channel (IC) with limited channel state information (CSI) feedback. We generalise two space-time interference alignment (STIA) algorithms to the K user MIMO IC with a transmit power constraint and show how these schemes exploit both current and outdated CSI to achieve an improved DoF performance. We show that by combining the STIA schemes with traditional methods major DoF gain can be achieved for a variety of limited CSI feedback scenarios in comparison to what would be obtained via the use of the traditional techniques on their own. In particular we prove that optimal DoF can still be achieved in situations where current CSI is unavailable for a derived fraction of the time. Finally we validate the promised DoF results via system simulation and show how these DoF gains reflect in terms of sum rate achievable.

IEEE Transactions on Communications, 2015

IEEE Access

IEEE Open Journal of the Communications Society

IEEE Transactions on Green Communications and Networking

2017 IEEE International Conference on Communications (ICC), 2017

IEEE Wireless Communications Letters, 2021

2020 IEEE 31st Annual International Symposium on Personal, Indoor and Mobile Radio Communications, 2020

IEEE Transactions on Green Communications and Networking, 2021

IEEE Transactions on Vehicular Technology, 2017

2016 IEEE 17th International Workshop on Signal Processing Advances in Wireless Communications (SPAWC), 2016

2016 IEEE International Conference on Communications Workshops (ICC), 2016

IEEE Transactions on Vehicular Technology, 2017

2015 IEEE Globecom Workshops (GC Wkshps), 2015

IEEE Transactions on Wireless Communications, 2015

2013 IEEE 24th Annual International Symposium on Personal, Indoor, and Mobile Radio Communications (PIMRC), 2013

2014 IEEE 25th Annual International Symposium on Personal, Indoor, and Mobile Radio Communication (PIMRC), 2014

IEEE Transactions on Communications, 2015